1. Field of the Invention
This invention relates to a solid electron emitting device having an NPN transistor structure and applicable to a flat display, an electron beam drawing apparatus or a CRT display, and more particularly to a solid electron emitting device having improved emission efficiency.
2. Description of Related Art
A typical electron emitting device is disclosed in J. Vac. Scv. Techonol. B4(1), 1986, P105. FIG. 4 shows an energy band of an NPN transistor structure disclosed therein. In this structure, electrons are injected from an emitter into a base region and some of the electrons passing through an extremely thin base are changed into thermoelectrons by an electric field between the base and a collector. Those electrons increase in kinetic energy, thereby being emitted into a vacuum.
However, in the above conventional art, since sufficient kinetic energy cannot be given to the electrons by only the electric field between the base and the collector, it is difficult to emit the electrons into the vacuum. An energy band of a device attempting to solve the above problem is shown in FIG. 5. Such a device aims to join semiconductors having different bandgaps between an emitter and a base and to form a heterojunction, so as to give kinetic energy to electrons with the use of the discontinuous width .DELTA.Ec of a band at the junction. However, according to this structure, since the heterojunction is formed in a depletion layer between the emitter and the base, .DELTA.Ec is smaller than .DELTA.Ec formed outside of the depletion layer, and thereby the emission efficiency of the electrons is low.